1. Field of the Invention
This invention relates to a silver/metal oxide material for producing electrical contacts which contains Ag as a main component and in which a metal oxide is distributed, and more particularly, to a material for producing electrical contacts which contains no Cd and to a method of producing this material.
2. Description of the Prior Art
Rationalization and automation in various industrial fields have been remarkably promoted in recent years, and related apparatuses tend to be increased in size and become complicated. In contrast, it would rather be necessary to reduce the size of a control system for such apparatuses and increase the frequency of its use and the capacity. Also the loads of its electrical contacts are now being increased.
A so-called Ag metal oxide material for producing electrical contacts consisting of Ag/and cadmium oxide distributed therein has superior contact characteristics in terms of welding resistance, errosion resistance and so forth and is specifically effective when used as a medium-load contact. On the other hand, materials containing no Cd have been developed on a turning point where the harmfulness and the problems of pollution resulting from refining of Cd were pointed out. It has been confirmed that materials made by distributing oxides such as those of Sb, Sn, Zn, In, Cu, Mn, Bi, Pb into Ag enable contact properties equivalent or superior to those realized by Ag/cadmium oxide materials and that they are therefore effective.
These silver/metal oxide material for producing electrical contacts are made on the basis of a sintering process or an internal-oxidation process, and they are mostly made by an internal-oxidation process at present.
In an internal-oxidation process, an alloy produced by melting from Ag and solute metals such as Cd, Sb, Sn and so forth is worked to have a desired shape, and this alloy is usually heated to a temperature higher than about 740.degree. C. at an oxygen partial pressure higher than 3 atm, thereby selectively oxidizing only the solute metals. This process entails a certain limitation in the compositional conditions which at least assures plastic working and internal oxidation.
This internal-oxidation process causes a concentration gradient of a solute metal in the alloy in the direction of the thickness thereof facing the direction of the diffusion of oxygen since, in this internal-oxidation process, oxygen is forcibly supplied from the outside so that solute metals in a solid phase are oxidized for a long time with oxygen diffused in the Ag matrix. This is extremely disadvantageous in terms of contact characteristics and is basically inevitable because of the mechanism of oxidation.
It is well known that, particularly in the case of oxidation effected from both surfaces of the material, unevenness of concentration is gathered to a central portion so that a layer where the densities of oxides are low is formed therein (depleted zone) (refer to Japanese Patent Publication No. 16505/1985). The thickness of this layer varies depending on the kinds and the concentrations of solute metals, the oxygen partial pressure and the internal-oxidation temperature, and it reaches even 0.1 to 0.3 mm, the contact characteristics thereby being greatly damaged.
According to this process, oxygen is diffused from the outside into the contact piece through the entire thickness thereof and, therefore, the greater the thickness, the longer the time for oxidation. This process also has disadvantages in terms of production control such as difficulty in the determination of the time when the oxidation is completed and a resultant high fraction defective.
Since the internal-oxidation process is effected by forcibly supplying oxygen into the material from the outside at a high temperature under a high pressure, a certain degree of strain remains after the completion of this process, and at the same time some increase in volume corresponding to the quantity of oxygen entering into the material is caused, resulting in internal defects such as fine cracks.
One of inevitable disadvantages of the internal-oxidation process resides in the existence of grain boundaries formed by agglomeration of oxides. The grain boundaries have extremely low electric and thermal conductivities and act to reduce the emanation rate of heat generated as joule heat or arc heat so that the contacts tend to accumulate heat, thereby causing a temperature rise thereof and, hence, increase in the amount of errosion.
Moreover, this internal-oxidation process has a fatal disadvantage in that the quantities and the kinds of solute metals relative to Ag are limited since it is difficult for oxygen to enter into the material to continue the internal oxidation if the content of solute metals exceeds a certain level.
On the other hand, powder metallurgy, which is also called a sintering method, is a generic name of methods in which Ag powder and base metal oxide powder are sintered or Ag powder and base metal powder are internally oxidized after they are sintered. It includes:
.circle.1 "Ag powder--oxide powder mixing sintering method" in which Ag powder and oxide powder or coprecipitation oxide powder formed from base metals are mechanically mixed and thereafter sintered;
.circle.2 "Sintering and internal-oxidation method" in which Ag alloy powder which is made by atomization and which is not yet oxidized is sintered and thereafter undergoes internal oxidation;
.circle.2 "Crushed piece internal oxidation sintering method" in which plates or wires formed after casting are crushed and small pieces thus formed undergo internal oxidation and are thereafter sintered;
.circle.4 "Internal oxidation and crushing sintering method" in which an Ag alloy formed after casting is worked into plates or wires and thereafter undergoes internal oxidation and Ag/metal oxide material thus obtained is mechanically crushed and sintered, and so forth. However, all of these methods other than Method .circle.1 utilize internal oxidation.
Method .circle.1 which is a typical type of powder metallurgy does not need any large-scale equipment for melting process and hhas an advantage in that it is possible to use various types of oxide powder without any limitation in terms of formation of an alloy and internal oxidation. However, it is basically impossible for this method to omit the process of mechanically and physically mixing Ag powder and metal oxide powder, and therefore, this method tends to cause segregation in relation to the composition and cannot realize a uniform sintering density since it is difficult for this method to uniformly mix the powder because of the difference between specific gravities, so long as the method is performed in the gravitational field. For this reason, this method is scarcely used at present.
Method .circle.2 inherits the defects of the internal-oxidation process itself. And further, method .circle.3 causes similar problems since it is necessary for method .circle.3 to effect internal oxidation at a low temperature as in the case of method .circle.2 in order to prevent from diffusion between the mutual small pieces. Accordingly, this method causes the formation of depleted zone in each small piece in which the content of oxides is very low, as in the case of the abovedescribed internal oxidation.
Method .circle.4 uses a very complicated process in which an Ag alloy is formed to plates or wires by melting, casting, forging and plastic working and in which the alloy is pulverized after perfectly oxidized in the manner of internal oxidation, thereby considerably increasing the production cost. Moreover, there is a certain limitation of processing in mechanical pulverization, and this method cannot reduce the particle size below 0.1 mm, and therefore cannot provide fine powder. Also there is a possibility of extraneous substances being mixed with the powder at the time of pulverization and affecting the characteristics of the resultant material. And further, a depleted zone formed at the time of internal oxidation may be broken but they remain in the mixture as coarse grains and affect the internal structure after sintering and cause unevenness of this structure, resulting in abnormal errosion.